Position-sensing apparatus

ABSTRACT

A numerically controlled machine tool has a rotatable spindle (10) to which is connected, in the place of the usual cutting tool, a probe (16) for measuring a work surface (25) to be, or having been, machined by a said tool. The probe (16) includes an electric circuit (30) adapted to respond to engagement between the probe (16) and the work surface (25), by disestablishing a signal (26) to a control system (12) and thereby effect measurement of the position of the work surface (25). The circuit (30) is energized by a battery (31) normally disconnected from the circuit (30) and connectable thereto by the control system (12) effecting a transient rotation of the spindle (10). The rotation acts on a centrifugally operated, normally open switch (FIG. 2) arranged within the probe (16). The circuit (30) is arranged to latch the switch in the &#34;on&#34; condition when the transient rotation ceases whereafter the probe (16) is ready for the probing operation. The latch is released responsive to said disestablishment of the signal (26) or by a subsequent transient rotation of the spindle (10). In this way the duration for which the battery is in use can be limited to the duration of the measuring operation and battery capacity is thereby saved.

BACKGROUND OF THE INVENTION

This invention relates to position-sensing apparatus and to a probe forsensing the position of an object in such apparatus.

It is known in numerically controlled machine tools to provide anautomatic tool change mechanism adapted to connect either a tool or aprobe to the operating spindle of the machine, the probe being requiredfor the purpose of measuring a workpiece machined by the tool. When atool is connected to the spindle the latter is then rotated to perform acutting operation but when a probe is connected to the spindle thelatter is held against rotation as is required for a probing operation.

The probe comprises an assembly including means defining an electriccircuit, a battery for energising said circuit, and means for changingthe state of the circuit responsive to the probe attaining a sensingrelationship with the workpiece. It is also known to provide theassembly with a switch for switching the battery on or off, the switchbeing operated to switch the battery on by engagement of a switchoperating member with the spindle when the probe is connected to thespindle, and the battery being automatically switched off when theswitch operating member becomes disengaged from the spindle as a resultof the probe being removed from the spindle. Thus the probe is armedonly when it is connected to the spindle and battery power is saved inthis way. However, there is still a waste of power because the probe isnot in use all the time it is connected to the spindle. It is an objectof this invention to reduce or overcome this difficulty.

SUMMARY The above and other objects are accomplished according to theinvention wherein the battery is switched on by rotation of the spindle,prior to the spindle being held against rotation for the probingoperation itself. Such a spindle rotation may also be used to switch thebattery off. In this way the time during which the battery is on can berelated to the probing operation itself rather than to the time theprobe is present on the spindle. BRIEF DESCRIPTION OF THE DRAWINGS

An example of position-sensing apparatus according to this inventionwill now be described with reference to the accompanying drawingswherein:

FIG. 1 is an elevational view in partial section and partial blockcircuit diagram form showing a part of a machine tool embodying theinvention.

FIG. 2 is a sectional view through the line II--II in FIG. 1.

FIG.3 is a circuit circuit schematic in partial block circuit diagramform showing an electrical circuit for use in the tool according to theinvention.

FIG. 4 is a circuit schematic in partial block circuit diagram formshowing an alternative circuit.

FIGS. 5 and 6 are views similar to FIGS. 2 and 4, respectively, showinga modification thereof.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIGS. 1, 2 the machine tool has a tool spindle 10 supportedfor rotation in a bearing housing 11. The rotation of the spindle iseffected by a program 13 connected to a control system 12 for the latterto output a miscellaneous function signal 14 to a motor 15 connected toone end of the spindle 10. The other end of the spindle 10 has removablyconnected thereto a probe 16 for sensing the position of a workpiece 17mounted on a table 18 which is movable relative to the spindle by anumerical control signal 19 output by the control system 12. Theposition of the table 18 is communicated to the system 12 by a feedbacksignal 20. The probe 16 is connected to and removed from the spindle 10by an automatic tool changer 21 operated by a miscellaneous functionsignal 22 from the system 12.

The probe comprises a housing 23 supporting a stylus 24. The position ofa surface 25 of the workpiece is sensed by moving the table 18 so thatthe stylus 24 engages the surface 25. Responsive to such engagement theprobe disestablishes a signal 26 which is transmitted from the probehousing 23 to the stationary structure defined by the bearing housing 11by means of an inductive coupling 27. The signal 26 is connected to thesystem 12 for the latter to record the position of the table, as read bythe signal 20, at the instant of the disestablishment of the signal 26and thereby record the position of the surface 25 of the workpiece 17.For the purpose of producing the signal 26, the stylus is supported onthe housing 23 by three normally closed electrical contacts 28 connectedin series in an electric circuit 30 (FIG. 3). When the stylus engagesthe workpiece it is displaced relative to the housing 23 and at leastone of the contacts 28 is broken thereby disestablishing the signal 26.The circuit 30 is energised by a battery 31 arranged within the housing23 and the circuit includes an oscillator 32 which generates thealternating current necessary for the coupling 27. The coupling 27itself comprises two spaced apart coils 27A,27B provided respectively onthe probe 16 and on the housing 11 for the contactless transmission ofthe signal 26 from the probe to the control system.

More specifically, the circuit 30 comprises four parallel limbs A,B,C,D.The limb A contains a normally open switch 33 for connecting anddisconnecting the battery 31 to the circuit 30. The switch 33 isactuable by an arm 34 (FIG. 2) which holds the switch 33 in the openposition under the bias of a spring 35. The arm 34 is pivotallysupported in the housing 23 in a position offset from the rotationalaxis, 10A, of the spindle 10 and such that, when the spindle is rotatedfor a short period, say one full rotation, the centrifugal force actingon the arm 34 causes the latter to withdraw from the switch inopposition to the spring 35 and allow the switch 33 to close and therebyestablish the connection between the battery 31 and the circuit 30.Further, the limb A contains a relay 36 having a normally open contact36-1 in the limb B, and the latter is connected to the limb A such thatwhen the contact 36-1 is closed the switch 33 is bypassed and the relay36 is latched in the "on" condition and remains in that condition afterthe spindle is stopped again.

The limb C contains the contacts 28 and a relay 37, all in series with anormally open contact 36-2 of the relay 36. The relay 37 has a normallyclosed contact 37-1 in the limb B in series with the contact 36-1.Further, the relay 37 has a normally open contact 37-2 in the limb D inseries with the oscillator 32 and the one coil, 27A, of the coupling 27.Thus the limbs C,D are normally de-energised, and they become energisedand the signal 26 is established responsive to the transient closure ofthe switch 33. The whole circuit 30 becomes de-energised and the signal26 is disestablished when, on operation of the probe, one of thecontacts 28 is broken. A delay element 38 may be introduced in the limbB to allow more than one probing operation to be carried out before afinal such operation de-energises the circuit 30.

In FIG. 4, the circuit 30 also comprises four limbs A,B,C,D. The limb Acontains the switch 33 and a first toggle-type flip-flop 40 having oneof its outputs, Q, connected as the input to a second toggle-typeflip-flop 41. The flip-flops 40,41 are connected to be energised whenthe switch 33 is closed. The flip-flop 40 has a pre-set circuit(not-shown) arranged to ensure that, when the switch 33 is closed, it isalways the output Q thereof which is raised to logical "1". Thussuccessive operations of the switch 33 cause the outputs Q,Q of theflip-flop 41 to become "1" alternately. As will be seen, this means thatthe circuit 30 can be energised and de-energised by successive rotationsof the spindle 10. The output Q of the flip-flop 41 is connected to arelay 42 having a latch contact 42-1 in the limb B. The output Q of theflip-flop 41 is connected to a relay 43 having a normally closed contact43-1 in the limb A such that when the output Q becomes "1" the relay 42is de-energised. The limb C contains the contacts 28 and a normally opencontact 42-2 closable by the relay 42. The limb D contains theoscillator 32 and the coil 27A in series with a normally open contact44-1 closable by a relay 44 in the limb C. It will be clear that theswitch 33 and the contacts 42-1, 42-2, and 44-1 normally maintain thecircuit in the de-energised condition and that successive rotations ofthe spindle respectively energise and de-energise the circuit 30, i.e.establish and disestablish the signal 26.

In use, let it be assumed that the workpiece 17 has been placed onto thetable, possibly manually, and that the position of the surface 25relative to a datum is known approximately. The first operation to beperformed by the system 12 is to more exactly ascertain the position ofthe workpiece. Accordingly the program 13 is written to perform thefollowing steps:

(a) operate the tool changer 21 to transfer the probe 16 from a magazine(not shown) to the spindle 10;

(b) operate the motor 15 to rotate the spindle 10 to arm the probe, i.e.establish the signal 26;

(c) move the table 18 to bring the stylus 24 into engagement with thesurface 25 of the workpiece;

(d) responsive to disestablishment of the signal 26, read the feedbacksignal 20;

(e) move the table 18 to withdraw the workpiece 17 from the stylus;

(f) as appropriate, operate the motor 15 to rotate the spindle 10 todisarm the probe;

(g) operate the tool changer 21 to remove the probe 16 from the spindle10.

It will be appreciated that following step (e) above, other surfaces ofthe workpiece can be sensed so that the three-dimensional position ofthe workpiece is ascertained. Following step (g), the program thenoperates the tool changer to replace the probe 16 by a cutting tool (notshown) and operates the machine to perform a machining operation on theworkpiece 17. Following the machining operation the program operates toreplace the tool by the probe and performs a measuring operation on themachined surface.

As an alternative to the lever 34, switch 33 and flip-flops 40,41, FIGS.5 and 6 show a cantilever spring 49 having a fixed end secured to thehousing at the axis 10A thereof, the spring 49 extending radially fromthe axis 10A to a free end to which is secured a mass member 54 and adouble-sided contact 50 situated between two contacts 51,52 secured tothe housing 23. When the spring 49 is free the contact 50 is clear ofthe contacts 51,52, and the contacts 50, 51,52 constitute a normallyopen two-way switch 53. It will be clear from the earlier description ofFIG. 4 that engagement of the contacts 50,51 connects the battery 31 tothe circuit 30 and engagement of the contacts 50,52 disconnects thebattery. The engagement of the contacts 50,51 and 50,52 is effected byrotating the spindle 10 respectively in the one and in the oppositesense of rotation. More specifically the battery 31 is connectable tothe circuit 30 by a manouvre comprising accelerating the spindle 10 in,say, the clockwise sense at a rate at which no contact engagement at allis effected and then decelerating the spindle at a rate sufficientlyhigh to effect engagement at the contacts 50,51. Disconnection of thebattery is effected by repeating the manouvre in the opposite sense. Thegradual acceleration and sharp decelaration of the spindle 10 isachieved by virtue of the fact that the spindle 10 is usually providedwith a brake (not shown) operable by the control system 12 and having arate of decelaration higher than the rate of acceleration of the motor15.

I claim:
 1. Probe for sensing the position of an object comprising:meansdefining an electric circuit; means for changing the state of thecircuit responsive to the probe attaining a sensing relationship withthe object; an electric power supply means for energising said circuit;switch means for connecting and disconnecting said power supply means tosaid circuit, characterised by; movement-responsive means responsive tomovement of said probe for acting on said switch means to establish aconnection between said power supply and said circuit; means responsiveto said connection being established between said power supply means andsaid circuit for maintaining said connection following cessation ofmovement of said probe; and means responsive to the changing of thestate of said circuit for disestablishing said connection.
 2. Probeaccording to claim 1, wherein said movement-responsive means is arrangedto respond to rotational movement of said probe about an axis of saidprobe.
 3. Probe according to claim 2, wherein said movement-responsivemeans is arranged to respond to centrifugal force generated by therotational movement.
 4. Probe according to claim 3, wherein saidmovement-responsive means includes a member supported in a positionoffset from said axis for radial movement in respect of said axis whensubjected to a centrifugal force, and said switch means is arranged tobe operated by said member responsive to radial movement thereof. 5.Probe according to claim 2, wherein said movement-responsive means isarranged to respond to angular acceleration or decelaration of saidprobe about said axis.
 6. Probe according to claim 5, wherein saidmovement-responsive means includes a member supported in a positionoffset from said axis for angular movement about said axis in responseto the acceleration or deceleration of said probe, and said switch meansis arranged to be operated by the angular movement of said member. 7.Position-sensing apparatus comprising:a spindle rotatable about an axis;probe means for sensing the position of an object, said probe meansbeing connected to said spindle and including:means defining an electriccircuit; means for changing the state of said circuit responsive to saidprobe attaining a sensing relationship with the object; an electricpower supply means for energising said circuit; switch means forconnecting and disconnecting said power supply means to said circuit;and movement-responsive means responsive to movement of said spindle foracting on said switch means to at least connect said power supply tosaid circuit.
 8. Probe for sensing the position of an objectcomprising:means defining an electric circuit; means for changing thestate of the circuit responsive to the probe attaining a sensingrelationship with the object; an electric power supply means forenergising said circuit; switch means for connecting and disconnectingsaid power supply means to said circuit, said switch meansincluding:means for establishing a connection between said power supplymeans and said circuit responsive to a first rotation of said probeabout an axis of said probe; means responsive to said connection beingestablished for maintaining said connection following cessation of thefirst rotation; and means for disestablishing said connection responsiveto a rotation subsequent to a cessation of the first rotation.
 9. Probeaccording to claim 8, wherein said means for establishing saidconnection and said means for disestablishing said connection arearranged to respond to angular acceleration or deceleration of saidprobe assembly about said axis.
 10. Probe according to claim 9, whereinsaid means for establishing said connection and said means fordisestablishing said connection comprise a member supported in aposition offset from said axis for angular movement about said axis inresponse to acceleration or deceleration of said probe, and said switchmeans is arranged to be operated by the angular movement of said member.